Most newer vehicles today come standard with a tire pressure monitoring system(TPMS), however many vehicles use an indirect tire pressure monitoring system that do not actually have sensors inside the tires to measure air pressure but detect low tire pressure by comparing relative wheel speeds via the anti lock brake system(ABS) wheel speed sensors. As a tire looses air pressure the diameter decreases slightly.
Most older indirect tire pressure monitoring systems are not as sensitive to change in tire pressure as the direct systems that actually monitor the air pressure inside the tire. An indirect TPMS are a fairly affordable add on to a vehicle already equipped with ABS. Federal law requires the TPMS to alert drivers if the air pressure inside the tires drops below 25% of the recommended pressure. The same rules apply to both direct and indirect systems causing many car manufacturers to switch to direct TPMS to meet the federal mandate.
One of the downsides to a indirect TPMS is that the warning light may not be triggered if all the tires are under-inflated because the tires are all equally under-inflated the wheels will all be spinning at the same speed and therefor the indirect system would not notice there is a problem.
However in recent years indirect systems have been making a come back starting in 2011 with all Audi models and in 2013 the Honda Accord, Mazda CX-5 and the VW Golf all switched to an indirect system. If you are getting warning lights or have questions about your TPMS sytems bring it by Orient Auto Service for a look. We are located in Gresham Oregon at 1550 SE Orient Drive, Gresham, OR 97080.